Detonation meter



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VIMIPNMMEL Patented Aug. 3l, 1948 DETONATION METER Raymond G. Piety, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application September 20, 1943, Serial No. 503,166

This invention relates to electrical measurement of physical forces, and it has particular relation to the measurement of peak values, and alternatively of average values, of the knocks caused by detonation of a fuel and air mixture in an internal combustion engine.

The primary object of my invention is to replace or supplement the human ear used in the prior art to determine when knocking is taking place by a standard electronic device not subject to human differences between individuals, nor human errore by providing suitable methods and apparatus.

Another important object is to provide an electronic device which will not only detect knocking but will give the degree of knock intensity.

Another object is to provide a detonation meter that will give the degree of knock intensity as a reading of the peak knock intensity during a short period.

Another object is to provide a detonation meter that will give the degree of knock intensity as an average of the knock intensity over a short period.

A further object is to accomplish the above listed objects without introducing any new factors into the prior art rating of the fuel but merely remove the variations introduced by the use of the variable human ear.

In the prior art the testing of fuels for knocking was done in a test engine,l the knocking being listened for by ear in conjunction with'observations on a cathode ray tube of the rate of change of pressure in the cylinder. This important test should not be made relying so much on the ear due to the impossibility of standardizing hearing in human beings. By substituting an electronic device, not only is more acute hearing obtained but the hearing is absolutely standardized so that different individuals running the test, or the same person on different days, will get identical results even though their hearing may vary greatly from time to time. The human ear cannot judge the absolute intensity of knocking. My electronic device can grade the knocking as to degree in an accurate manner.

Other objects are to provide methods and apparatus useful in signalling and indicating.

Numerous other objects will be apparent to those skilled in the art upon reading the accompanying specification and claims and studying the drawings.

In the drawings:

Figure 1 is a schematic diagram of thel functional units of an illustrative embodiment of my invention, with voltage-time diagrams of the cur-v 16 Claims. (Cl. 73-35) differences in pressure are generated within the cylinder. These pressure differences are propagated as sound waves within the cylinder-.and they are reflected back and forth until their energy is dissipated by adsorption and transmission through the cylinder walls. If the pressure of the gas in the cylinder is measured by some type of pressure indicator, it will be found that the effect of these reflections back and forth within the cylinder and also within the solid walls is to select out of the original exciting pressures short trains of superposed damped sine waves.

The frequency of oscillation of each of thev damped sine waves corresponds to one of the modes of vibration of the system.

There are a great many modes in a system like a test engine. In the case of my detonationA meter, I use the modes with frequencies between 12,000 and 17,000 cycles. The ping which is heard when the engine is knocking is nothing more than the sound generated by the excitation of some of these normal modes between 12,000 and 17,000 cycles.

In the case of a test engine, the normal modes have rather high frequencies of oscillation, and it turns out that in order to generatie a pressure variation with appreciable energy at these frequencies, a very rapid rise in pressure is required. The presence of mechanical oscillations of high"frequency (several thousand cycles pe second) is therefore an indication of a rapid rise in pressure in the cylinder. This rapid rise of pressure is also an indication of knocking, and it is this rapid pressure change that generates the voltage in the pick-up 4 of my device. The more rapid the rise of pressure, the more violent the knock and the greater-the voltage generated by the pickup, so I can thereby determine the dethe acoustic and mechanical vibrations set up, it

has been observed that' the intensity of the knock` under controlled conditions can be used to compare one fuel with another.

These are several pressure sensitive devices which are able to transform the pressure variations within an engine cylinder to a corresponding electrical voltage. I have been using a very satisfactory magneto-striction type indicator manufactured by the Standard Oil Company of California the voltage output of which is substantially proportional to the rate of change of pressure. This indicator has been used in the prior art in conjunction with an amplifier and a cathode-ray tube to give a visual pattern of the pressure variations in the cylinder. The presence of knock is clearly indicated by the pattern on the cathode ray screen, but it is difficult to observe the intensity of the knock, and impossible in testing practice to measure its value. Knock intensity is best measured 'by my present invention.

In order to use a meter deflection to measu-re the intensity of the knock, it is necessary to remove the effect of valve clatter Wh'ich also sets up high frequency vibration of the cylinder walls and electro-magnetic waves from the spark plug which generate static currents in the wires of the Imeter. Fortunately the valve clatter and ignition spark occur at different times than the detonation so that by an electronic switching action it is possible to remove the effect of valve clatter and ignition spark. After removing the ef- A l fects of valve clatter and spark, the question re' mains as to how the meter should be made to respond to the output of the indicator. The combustion' cycles of the test engine are extremely variable. When tests are made by car the presence of an audible knock once every thirty c'ombustion cycles is considered to indicate knock. Observation of the cathode ray pattern showed that intermediate cycles might only indicate an intensity of ten per cent of the knocking cycle.

I may measure the time average knock intensity, or a time average where the weight given to a knock of any given intensity is proportional to some suitable function of the knock intensity, and/or the time elapsed in each cycle.

For example, when using an averaging meter 38 shown in Figure v1 the meter deflection is made to average the knock intensity over a sh-ort period of time, giving the same weight to all knocks.

I preferably employ a meter 31 shown in Figure 1 which is designed to give an indication of the peak knock intensity. This has been accomplished by using a high speed critically damped meter in conjunction with an electronic circuit which would prolong the peak voltage due to knock just sufficient to make it easy to read the maximum meter deflection. This gives a meter which will follow the variations in knock intensity and clearly sh-ow the statistical fluctuations between combustion cycles. The sensitivity of the meter is adjusted so that a deilect'ion in the middle of the scale corresponds toaudible knock and with the action just described this deflection in effect corresponds to a calibrated ear.

In Figure 1', a test engine 3 is provided with a pick-up 4, preferably a pressure indicator pick-up of the magneto-striction type 4manufactured by the Standard Oil Company of California. This pick-up translates the pressure changes within the cylinder of the engine 3 to electrical energy by virtue of the change in the reluctance of a magnetic path caused by movement of a small diaphragm (not shown) in the cylinder wall, the

vvoltage output .being directly proportional to the rate of change in pressure. Internal combustion engines are almost always provided with poppet valves (see valve stem 5) and these poppet valves clatter in closing causing knock-type vibrations, 5 and due to inertia of parts in the pick-up 4 these spurious pressure change knock-type valve clatter vibrationsarevpicked up too. The vspark of ignition also sends out an electro-magnetic field which generates spurious pressure change" currents in the pick-up network of wires.

All the output of the pick-up, regardless of its source is transmitted along a cable 6 to an amplifier 8, and the shape of the wave transmitted along vthe cable 6 is shown roughly by a diagram 9 in which time is plotted in a horizontal direction (from left to right) and voltage in a vertical direction (above the line being positive and below the line negative). (This Wave is better seen in a diagram I0 after amplification.) As to be seen in the diagram -I0 there is a train of small amplitude waves consisting of spurious pressure change waves which are' really intake valve clatter II, ignition spark waves 1 and other vibrations it is not desired to finally indicate, then a high rate of pressure change wave I2 of large amplitudewhich is caused by the explosion of gas and resulting pressure rise and.fall during the power stroke.

On the crests of this pressure change Wave are the minor pressure ch-ange variations I3 and I4 which indicate the presence and degree of knocking. Variation I5 are exhaust valve clatter during the closing othe exhaust valve.

The exact shape of 1, Il, I3, I4 and'IS cannot be shown as 12,000 to 17,000 cycles per second would just be a blurusing the scale employed in the drawings.

This wave trainds amplified in its entirety in the amplifier 8 and is transmitted through a cable I6 to a tuned amplifier I1. The shape of the amplified wave is shown by the diagram I0, all of these diagrams being similar to the diagram 9 as to time and voltage relations, each base line starting and ending at the same point in the engine' cycle.

Cables 6 and I6 in combination with the ground form channels for the waves, Iand therefore the cables in Figure 1 can all be referred to as channels.

The same amplified wave train I0 is transmitted along a channel -IB to an integrator I9. 'I'he integrator integrates and modifies the wave train of the diagram I0 into a pulse of the general shape shown in a diagram 20 and this pulse is transmitted along a channel 2| to a clipper 22. The clipper 22 squares up the pulse 'into the substantially square pulse shown in a diagram 23. The

gie of squareness can be varied to any extent considered necessary and the ignition wave 1 even if encroaching on slope I2 will be outside of the square pulse after clipping. This square pulse is transmitted from the clipper 22 to rectifier and -mixer 24 by a channel 25.

The tuned amplifier I1 preferably amplifles the waves shown in the diagram I0 that constitute a 5 pass band of about 12,000 to 17,000 cycles per second. This amplified pass band 21 is to be combined with the square pulses sh-own in the diagram 23 in the mixer portion of the rectifier and mixer 24 and is transmitted to the rectifier and mixer by an electro-magnetic field forming a channel represented by spark symbols 28.

In the rectifier and mixer 24, the wave trains 21 and 23 arecombined to form the wave train shown in a, diagram 29. 'I'his c'ombined wave train is sent over a channel 30 to a network 3l which either 'averages or generates a sustained peak voltage depending on the position of a switch 39 The averager or sustained peak voltage generator 3| operates to transmit a voltage over a channel 32 to a meter box 33 having the general wave form of a diagram 34, in which a rapid rise 35 is followed by a relatively slow exponential decay 38. Actually a composite wave is for-med as a series oi' these waves 34 are superposed on each other at short intervals.

The meter box 33 contains two meters 31 and 38. 81 being a peak voltage meter and 39 an average'voltage meter. A gang switch composed of an electrically nonconductive control rod 39 and singie pole double throw switches 49, 4| and 42 may be provided to switch (by switch 48) from the meter 31 to 38 and to switch (by switches 4| and 22) the generator 3| from sustained peak voltage generation for the lmeter 31 to generation of voltages proportional to those of wave 29 for averaging by the meterr 38. Contacts 43 to 5| inclusive are provided for the switches.

Figure 2 shows a wiring diagram of an illustrative embodiment of my invention.

Any power supply means may be employed at 59 for supplying the tubes with the type current they require. For illustration, I show a transformer -receiving house current at 68, having a core 81,

coils t8, 89 and G9 and a rectifier 8|, but any other rpower supply can be employed just as well.

In the pick-up 4, a grid leak 62 of 1.0 meg. between wires 8 and 5 and a ground 93 for the lwire 8' is provided. This grid leak and ground could be regarded as part of the amplifier also, as they control the grid of tube 84 and prevent excessive plate to cathode currents, the exact dividing line between elements having certain functions being of course vague, although the elements and their functions are not vague. While I have shown certain d'otted lines around units in Figure 2, it should be understood a certain latitude exists as to exactly what elements constitute each unit.

The amplifier 8 shows two triode tubes 84 and 88, for simplicity but I may use a single double triode tube in place of these two tubes. The amplifier 8 is shown as a conventional resistance coupled amplifier. Any other type amplifier could be substituted.

The integrator i9 preferably contains a resistance 98 of about 0.5 meg. in series with a condenser 81 of about .25 mfd. By electric circuit theory the voltage between a wire 89 and the ground is substantially the integral of the signal from the wire I9. This voltage drop across the condenser 61 is impressed on a grid wire 98 of a triode 99. Again I may use a single tube in place of triodes 69 and 18. Other circuits prod-ucing similar integration may be used.

The clipper 2'2 receives the output from the tube 89 which is controlled as to volume by a volume control 1|, and sent to the grid of a triode i8. This triode is operated without grid bias and with a .5 meg. resistance 12 in the grid circuit which serves to clip .the positive voltages and take the rounded top oi of wave pulse 29. A resistance 13 of 1.0 meg. and a double diode 19 act as a first stage voltage limiter and a resistance and a double diode 18 act as a second stage voltage limiter of a conventional clipping circuit for the production of the rectangular wave pulses 23. I prefer to take double triodes and make double diodes out of them by wiring the grids directly to the plates. Other clipping circuits may be substituted. The number of tubos is such that the phase of the square pulse is positive with respect to the ground.

The tuned amplifier I1 is conventional. I preferably employ a loosely coupled tuned, air core, or iron dust core, transformer as my band pass filter composed preferably of primary coil 11 and secondary coil 18 and condensers 19 and 88.

- Any other of the many band or high pass filters could be employed. Tube 8| is a part of the tuned amplier |1 with a rheostat 82 controlling its gain, and thereby controlling the amplitude of the waves 2l. A v y The rectifier and ,mixer 24 could have been shown as two units, one unit mixing and the other unit acting as a modulated rectifier. Because oi' the close relation between them. they are shown as a single unit, but because of their separate functions they may be claimed as two separate units.

The mixer portion of the rectifier and mixer 24 consists of a mixing tube 88 in which the voltages of the wave form 21 and the square wave form 23 are added algebraically. The amplitude oi the wave 27 is controlled by the rheostat 82, the width of the wave 23 is controlled by the rheostat 'Il and the amplitude of the wave 23 is controlled by a rheostat 8i. n The potential between the cable 25 and the ground follows the square pulse wave pattern 23, and makes the grid of the tube 83 more positive at intervals to cause more plate-to-cathode current in tube 83, and this square pulse current of the tube 83, in passing through coll 18 and condenser 89 has the wave form 21 impressed on it from the primary coil 11. This combined wave train is impressed on the grid of a triode 85.

The rectifier portion of the rectifier and mixer 24 consists of the triode 85 which only transmits the more positive portion of the combined wave form. This is due to a bias placed on the cathode of the rectifier tube 85 by a wire 86. The wire 86 goes to a voltage divider 81 where the B2+ at one end and a resistance 88 and the ground at the other end are so disposed as to make the grid in the tube 85 more negative than the cathode in this tube to prevent the flow of plate current at all times except when the high portion of the square wave makes the grid in 85 more positive than the cathode in that tube. So only the tops of the square wave 23 with wave 21 impressed thereon are transmitted as wave 2 along the cables 38 and 52. I

I may use a single envelope containing the two triodes 93 and 95. While the rectifier and mixer portions of 24 are closely tied together it is obvious that they have separate functions, and other mixers and/or rectiiiers could be substituted.

The cable 82 transmits the wave 29 to the cathode ray oscillograph tube 53 where vit maybe observed on screen 54.

The cable 38 transmits the wave 29 to generator 3|.

The generator 3| will first be described with the switch 39 to the left so that the contacts 4-8 and 4|; 49 and 4'2; and 50 and 40 respectively are in contact. In this case the network 3| is a sustained peak voltage generator. Positive current of the wave form 29 runs from the cathode of the triode 95 through a plate 99 and a cathode 98 of a double diode tube 9| to charge a condenser 92. In the circuit shown in Figure 2 I have used a double triode connected as diodes in order to obtain suiiicient conductance with 7 the low cost radio receiver type tubes to rapidly charge condensers 92 and 95. e

The condenser 92 is preferably of about 0.05 mid. capacity and canI thereforelbe charged in less than 0.001 of a second by the. rectified signal wave 29, so that the peak voltage on tliis condenser is closely proportional to the peak signal voltage of 29. As a resistance 93 of about 2.0 meg. is all that tends to short the condenser, the duration of the peak voltage o n the condenser 92 is appreciably longer than the signal peak so that when the voltage of the condenser 92 is impressed through 43, 4I 46 on the grid of a triode 94, the resulting platc-to-cathode current in the tube is able to charge a condenser 95 of about 2.0 mfd. capacity also shunted by a resistance 96 of about 2.0 meg. which condenser therefore has an increased time constant and maintains its peak voltage to control the grid of a triode 91. The plate-to-cathode current of triode 91 makes the high speed peak meter 31 4read the highest peak of the wave train 29 with a slow enough exponential decay of voltage between successive highest peaks so that the human eye can read the high value as the meter hand falls away slowly during the relatively .slow discharge of condenser 95. A rheostat 9B provides a zero a-djustment for the meter 31.

When the generator 3| is used as an averager, the switch 39 is to the right so that the contacts 4I and 41; 42 and A9; and 40 and 5I are lin contact. Then the voltage of the small condenser 92 is applied to the grid of the triode 91 and the -plate-to-cathode current oftriode 91 makes the high speed averaging meter l38 read the average of the peaks of the wave train 29 for the last few seconds as they appear across condenser 92 and resistor 93.

Operation I The pick-up 4 picks up the'wave 9 which includes the valve clatter, ignition spark electromagnetic waves, and the 'rate of pressure change valves includingthe high 'frequency rat of pressure change waves. These are all sent to the ampliiier which amplies them 'all in alinear man- It will be kseen that the output I of amplifier 8 is proportional to the rate of change of pressure I2, which includes the high ffrenuency os cillations I3 and I4 due to knock, 1 dueto spark ignition and I I and I due to valve clatter. This voltage I0 is impressed on an integrating circuit I9 which by the mathematical process of integration (performed electrically of course) integrates the rate of change of pressure Aplotted against time of I0 into the pressure plotted against time of 20. The clipper 22 then makes a rectangular wave 23 out of 20 by amplifying to steepen the sides and by cuttingoi the top of 20, This rectangular voltage-time wave is used to bias the rectifier 85 (Figure v2) in'the averager or sustained peak voltage generator so that it is operative only during the interval when considable vpressure exists in the cylinder. This operation removes the voltages 1 :due to ignition spark and Il and I5 due to valve vclatter since these voltages appear only when the pressure is low. The parallel channel I6, 28 with, the band: pass filter 11, 18 in the tuned amplier l14 removes all .the voltages from I0 except21 Which-are those corresponding to the excitation o'f highrfrequency modes of vibration in the interval between 12,000 and 17,0 00 cyclesl per second. `The waves 21 are only measured at times when the cylinder pres-y sure is high dueto the bias 88 on rectifier 85 and this eliminates the valve clatter the spark waves and other disturbances occurring when the pressure is lower. Then a' function of the peak volt. ages or the sustained peak voltage of the measl ureclx portion 21 is produced in wave 34 generated in generator 3|,` depending on the position of the switch 39. When 39 is to the left, sustained peak voltage is generated and measured; when it -is allel channel network for the waves, an integrator to the right, peak voltages are generated and the average thereof is measured. This voltage wave 34 that is measured has exponential decay 36 but in a short interval 4is replaced by anew wave. Meter 31 thus indicates the Peak, or largest knock in the last few seconds, when it is connected. When meter 38 is connected, it indicates the average knocking oi' the last few seconds.

As pointed out above many changes of circuits following the standard circuits old in the art, and many substitutions of different tubes, conventional elements or units having the same function, may be done by those skilled in the art without involving invention, thelscope of my invention being set forth in the following claims.

Having described my inventionl I claim:

1. The method of measuring the knock intensity during periods of detonation in an internal combustion engine cylinder, comprising the steps of generating a. first electrical Wave the amplitude-ofv which is in synchronism with and 4substantially proportional to the rate of change of pressure in the cylinder, transmitting the first wave in two channels of a parallel channel network, substantially integrating the rst wave into a second wave substantially comprising pulses having limited portions of uniform and substantially greater amplitude in synchronism with the periods of detonation in one channel of the parallel network, filtering out of the first wave a third wave comprising substantially a selected wave band in the other channel of the parallel network, adding the amplitudes of the secondand third waves and measuring a function of the peak amplitude of. the combined second and third waves.

. 2. A detonation indicator for indicating the presence of knocking in electrical waves which.

are proportionalto the rate of change of pressure in an internal combustion cylinder, comprising in combination an amplifier for the waves, a parand a square wave clipper to integrate the wave into pulses in one channel of the network, a band pass lter to pass a wave band from 12,000 to 17,000 cycles in the other channel of the network, a mixer to add the pulses t-o the wave band, a rectifier to bias out the wave band between the pulses, and electronic means to indicate the peak values of the added pulses and wave band.

3. A detonation indicator for indicating the presence of knocking in electrical Waves which are proportional to the rate of change of pressure in an internal combustion cylinder, comprising in combination a parallel channel network for the waves, an integrator and a square wave clipper to integrate the wave into pulses in one channel of the network, a band pass filter to pass a wave band in the other channel of the network, a mixer to add the pulses to the Wave band, a rectifier to bias out the wave band between the pulses, and electronic means to indicate the peak values of the added pulses and wave band.

4. A detonation indicator for indicating the presence of knocking in electrical waves which are proportional to the rate of change of pres- 9, sure in an internal combustion cylinder, comprislngiin combination a parallel channel network fo'r the waves, an integratorto integrate the wave into pulses in one channel of the network, a band pass iilter to pass a wave band in the other channel of the network, a mixer to add the pulses to the wave band, a rectier to bias out the wave band between the pulses, and electronic means to indicate the peak values of the added pulses and wave band.

5. A detonation indicator for indicating the presence of knocking in electrical waves which are proportional to the rate of change of pressure in an internal combustion cylinder, comprising in combination a parallel channel network for the waves, an integrator to integrate the wave into pulses in one channel of the network, a band pass lter to pass a wave band in the other channel of the network, a mixer to add the pulses to the wave band, and electronic means to indicate the peak values of the added pulses and wave band.

6. A detonation indicator for indicating the presence of knocking in electrical waves which are proportional to the rate of change of pressure in an internal combustion cylinder, comprising in combination a parallel channel network for the waves, an integrator and a square wave clipper to integrate the wave into pulses in one channel of the network, a band pass illter to pass a wave band from 12,000 to 17,000 cycles, in the other channel of the network, a mixer to add the pulses to the wave band, and electronic means to indicate the peak values of the added pulses and Wave band.

7. A detonation indicator for indicating the presence of knocking in electrical waves which are proportional to the rate of change of pressure in an internal combustion cylinder,com prising in combination yan amplier for the waves, a parallel channel network for the waves, an integrator to integrate the wave into pulses in one channel ofthe network, a band pass filter to pass a wave band in the other channel of the network, a mixer to add the pulses to the wave band, a rectier to bias out the wave band between the pulses, and electronic means to indicate the peak values of the added pulses and wave band.

8. In a detonation meter, means for converting pressure variations in a cylinder to electricall voltages, a threshold device forfrejecting components of less than a predetermined magnitude in said voltages, a circuit fed by said threshold device for generating exponential pulses of relatively short duration, a second. pulse generating circuit fed by said rst circuit for producing overlapping pulses of relatively long duration whereby a succession of said overlapping pulses produces a wave having a crest of saw-toothed coniiguration, and a high speed meter for measuring the output of said second pulse generating circuit.

9. In a detonation meter, means for converting pressure variations in a cylinder to electrical voltages representative thereof, a threshold device for rejecting components of less than a predetermined magnitude in said voltages, a pulse generating circuit comprising a condenser and a resistor for slowly discharging said condenser, whereby said circuit produces spaced, exponen- 4ial pulses in response to the voltage components passed by said threshold device, a second pulse generating circuit fed by said iirst circuit including a second condenser and a second resistor for slowly discharging said condenser, the time constant of the second resistance-capacitance unit being substantially higher than the time constant of the first unit, and a critically damped voltmeter for measuring the output of said second pulse generating circuit.

10. In a detonation meter, means for converting pressure variations in a cylinder to electrical voltages representative thereof, a threshold device for rejecting components of less than a predetermined magnitude in said voltages, a' circuit fed by said threshold device for generating exponential pulses of relatively short duration, a second pulse generating circuit fed by said rst circuit for producing overlapping pulses of relatively long duration whereby a succession of said pulses produces a wave having a crest of saw-toothed configuration, and means including a meter responsive to the output of said second pulse generating circuit.

, l1. In a detonation meter, means for converting pressure v-ariations in .a cylinder toy electrical voltages including a pickup and a lter for attenuating components cf predetermined frequent!A lcircuit comprising a unit including a second energy storage device and means for slowly discharging said storage device, the time constant of the second unit being substantially higher than the time constant of the first unit, and means including a voltmeter responsive to the pulses produced by said second circuit.

12. In a detonation meter, means for converting pressure variations in a cyclinder to electrical vvoltages including a pickup and a high-pass filter for passing frequencies above 12,000 cycles per second, a threshold device for rejecting voltage components of less than a predetermined magnitude in the output olf said converting means, a first pulsing circuit comprising a unit including an energy-storage device and means for discharging said energy storage device whereby said'circuit produces spaced exponential pulses in response to the voltages passed by said threshold device, a second pulsing circuit fed by said first circuit comprising a unit including a second energy storage device and means for slowly discharging said storage device, the time constant of the second unit being substantially higher than the time constant of the rst unit, and means including a' voltmeter responsive to the pulses produced by said second circuit.

1 3. In a system for measuring peak voltages, the combination comprising `means for converting pressure variations" into an'electrical voltage representative thereof, said converting means including a filter for suppressing undesired components of said voltage, a biased thermionic device connected tosaid converting means for passing voltages above. a predetermined value, means for converting said transmitted voltages into pulses of exponential form, means connected to said last means for generating sustained pulses having peak values proportional to the'peak values of said transmitted voltage, and indicating means operated by said sustained pulses.

14. In a detonation meter, the combination 'ing components thereof below a predetermined value, means for converting the output of said amplifier to a pulse having an exponential form, means for converting said exponential pulse into a sustained' pulse, and means for indicating the voltage variations of said sustained pulses.

15. In a system for measuring peak voltages, the combination comprising means for converting pressure variations into an electrical voltage representative thereof, said converting means including a filter for suppressing undesired components of said voltage, a biased electron discharge device connected to said converting means for passing voltages above a predetermined value, means for converting said transmitted voltages into pulses representative thereof, means connected to said last means for generating sustained pulses having peak values proportional to the peak values of said transmitted Voltage, and indicating means operated by said sustained pulses.

16. In a detonation meter, the combination comprising means for converting pressure variations into electrical voltages, said converting means including a circuit for filtering said voltages to eliminate undesired components thereof,

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number A Name i Date 2,174,176 Journaux et al. Sept. 26, 1939 2,275,675 Draper et al Mar. 10, 1942 2,287,926 Zepler June 30, 1942 2,313,666 Peterson Mar.l 9, 1943 2,337,522 Eldredge Dec. 2l, 1943 2,340,714 Traver et al Feb. 1, 1944 2,355,088 Lavoie Aug. 8, 1944 FOREIGN PATENTS Number Country Date 517,486 Great Britain Jan. 3l, 1940 553,523 Great Britain May 25, 1943 

